Surface molecular dynamics simulation with two orthogonal surface steps: how to beat the particle conservation problem

Due to particle conservation, Canonical Molecular Dynamics (MD) simulations fail in the description of surface phase transitions involving coverage or lateral density changes. However, a step on the surface can act effectively as a source or a sink of atoms, in the simulation as well as in real life. A single surface step can be introduced by suitably modifying planar Periodic Boundary Conditions (PBC), to accommodate the generally inequivalent stacking of two adjacent layers. We discuss here how, through the introduction of two orthogonal surface steps, particle number conservation may no longer represent a fatal constraint for the study of these surface transitions. As an example, we apply the method for estimating temperature-induced lateral density increase of the reconstructed Au (001) surface; the resulting anisotropic cell change is consistent with experimental observations. Moreover, we implement this kind of scheme in conjunction with the variable curvature MD method, recently introduced by our group.Comment: 9 pages, 5 figures, accepted for publication in Surface Science (ECOSS-19

Realistic grand canonical Monte Carlo surface simulation: application to Ar(111)

Most realistic, off-lattice surface simulations are done canonically--- conserving particles. For some applications, however, such as studying the thermal behavior of rare gas solid surfaces, these constitute bad working conditions. Surface layer occupancies are believed to change with temperature, particularly at preroughening, and naturally call for a grand canonical approach, where particle number is controlled by a chemical potential. We report preliminary results of novel realistic grand canonical Monte Carlo simulations of the Lennard-Jones (LJ) fcc(111) surface, believed to represent a quantitative model of e.g. Ar(111). The results are successful and highly informative for temperatures up to roughly 0.8 T_m, where clear precursor signals of preroughening are found. At higher temperatures, convergence to equilibrium is hampered by large fluctuations.Comment: 4 pages, REVTeX, 3 PostScript figure

Islands, craters, and a moving surface step on a hexagonally reconstructed (100) noble metal surface

Deposition/removal of metal atoms on the hex reconstructed (100) surface of Au, Pt and Ir should present intriguing aspects, since a new island implies hex -> square deconstruction of the substrate, and a new crater the square -> hex reconstruction of the uncovered layer. To obtain a microscopic understanding of how islands/craters form in these conditions, we have conducted simulations of island and crater growth on Au(100), whose atomistic behavior, including the hex reconstruction on top of the square substrate, is well described by mean s of classical many-body forces. By increasing/decreasing the Au coverage on Au(100), we find that island/craters will not grow unless they exceed a critical size of about 8-10 atoms. This value is close to that which explains the nonlinear coverage dependence observed in molecular adsorption on the closely related surface Pt (100). This threshold size is rationalized in terms of a transverse step correlation length, measuring the spatial extent where reconstruction of a given plane is disturbed by the nearby step.Comment: 11 pages, 5 figures, accepted for publication in Surface Science (ECOSS-18

Interfacial phenomena in molten metals-refractory borides systems

Non-oxide ceramics, such as carbides, nitrides and borides represent one of the fastest growing classes of new advanced materials. Among them, transition metals ceramic diborides, in particular Titanium, Zirconium and Hafnium diborides, are members of a family of materials with extremely high melting temperatures, high thermal and electrical conductivity, excellent thermal shock resistance, high hardness and chemical inertness. These materials -Ultra High Temperature Ceramics (UHTCs)- constitute a class of promising materials for use in high performance applications, where high temperatures, high thermal fluxes, severe surface stresses are involved. However, the possibility to exploit commercially their peculiar characteristics often depends to a great extent on the ability to join the ceramic parts one to the other or to special metallic alloys. As the behaviour of a metal-ceramic joint is ruled by the chemical and the physical properties of the interface, the knowledge of wettability, interfacial tensions and interfacial reactions is mandatory to understand what happens at the liquid metal-ceramic interface during joining processes. Provided that a large number of ceramic materials are not wet (or poorly wet) by pure liquid metals, their wettability by liquid-metal systems can be significantly modified by using either non-reactive metallic solutes capable of adsorption at the metal-ceramic interface, or reactive elements, so that the energetic contribution coming from reaction (and dissolution) free energy release could contribute to lower the total interfacial energy of the solid-liquid system, increasing, at the same time, and thermodynamic adhesion. Recent data on the wettability and the interfacial characteristics of different metal-ceramic systems, and in particular of (Ti,Zr,Hf)B2 in contact with liquid Ag and its alloys (Cu, Ti, Zr, Hf) are reported and discussed as a function of time, compositions and structure of the ceramic and of the alloy involved. In particular new data are shown about the interactions of Ag, Cu and Au in contact with ZrB2. Models are also used to interpret the wetting behaviour and the adsorption/reaction interfacial phenomena involved

Role of interface coupling inhomogeneity in domain evolution in exchange bias

Models of exchange-bias in thin films have been able to describe various aspects of this technologically relevant effect. Through appropriate choices of free parameters the modelled hysteresis loops adequately match experiment, and typical domain structures can be simulated. However, the use of these parameters, notably the coupling strength between the systems' ferromagnetic (F) and antiferromagnetic (AF) layers, obscures conclusions about their influence on the magnetization reversal processes. Here we develop a 2D phase-field model of the magnetization process in exchange-biased CoO/(Co/Pt)xn that incorporates the 10 nm-resolved measured local biasing characteristics of the antiferromagnet. Just three interrelated parameters set to measured physical quantities of the ferromagnet and the measured density of uncompensated spins thus suffice to match the experiment in microscopic and macroscopic detail. We use the model to study changes in bias and coercivity caused by different distributions of pinned uncompensated spins of the antiferromagnet, in application-relevant situations where domain wall motion dominates the ferromagnetic reversal. We show the excess coercivity can arise solely from inhomogeneity in the density of biasing- and anti-biasing pinned uncompensated spins in the antiferromagnet. Counter to conventional wisdom, irreversible processes in the latter are not essential

A Concerted Variational Strategy for Investigating Rare Events

A strategy for finding transition paths connecting two stable basins is presented. The starting point is the Hamilton principle of stationary action; we show how it can be transformed into a minimum principle through the addition of suitable constraints like energy conservation. Methods for improving the quality of the paths are presented: for example, the Maupertuis principle can be used for determining the transition time of the trajectory and for coming closer to the desired dynamic path. A saddle point algorithm (conjugate residual method) is shown to be efficient for reaching a ``true'' solution of the original variational problem.Comment: 3 figures, accepted for publication in Journal of Chemical Physic

Net electrophilicity as computational route for the choice of favorable ionic liquids in nanoparticle production

In the last years, the potential of using ionic liquids (IL)s as an environment for nanoparticle (NP) synthesis has been demonstrated and in particular, triggering NP formation in ILs by electron irradiation has been reported as a very simple and clean route for NP production. Starting from the recent evidence for a correlation between an IL's capability to support NP production and the radiochemical instability of the IL's cation, we used conceptual Density Functional Theory (DFT) to provide a pre-screening of a set of different IL cations. The screened quantity is the net electrophilicity which we suggest as possible measure of this instability. Therefore, our work not only gives a measure for the likelihood of NP generation in different ILs, but it also provides a model which can further be extended and applied to obtain information about any other IL of interest. Moreover, our theoretical approach outlines a strategy which may reduce a lengthy experimental investigation for the identification of the most suitable IL for a particular reaction.Comment: 12 pages, 11 figures, 2 tables (including SI